Automated placement of semiconductor bars on lead frames is presently accomplished using pick-and-placement mechanisms. This method may be briefly described as follows. A semiconductor bar is indexed to a known location, often through the use of a video camera alignment system. The semiconductor bar is then pushed of from a slice holder by a `Z` stage operated from beneath the slice. A semiconductor bar is captured by a pick-up tip and held there by a vacuum and is removed from the slice holder. The `Z` stage is them withdrawn leaving the semiconductor bar firmly held to the pick-up tip by the vacuum.
The pick-up tip and related mechanism is rotated up to 180 degrees and positioned above the area on a lead frame to which the semiconductor bar is to be mounted. After a mechanical settle-time, the pick-up tip is lowered to place the semiconductor bar in contact with the pad area of the lead frame. The vacuum is released and the semiconductor bar adheres to the bar pad area. The pick-up tip is then withdrawn for another cycle.
The pick and place method is a blind placement method. Once the semiconductor bar is secured to the tip of the pick-up time. The tip is rotated and the semiconductor bar is place on the lead frame, the accuracy of the placement depends upon the alignment of the pick-up tip with the indexed position of the lead frame.
The though put of such systems is in the range of 1000 to 3000 semiconductor bars mounted per hour. Systems are designed such that bar movement and slick-to-lead frame tolerances are minimized by careful set-up and good mechanical stability. However, as semiconductors slices become larger, the radius of the pick-and-place mechanism must increase requiring greater accuracy.